It has long been known that cathode ray tubes do not provide a light intensity that is linearly proportionate to a voltage applied to the tube. Instead, the intensity is proportional to the value of the applied voltage raised to an exponential factor gamma. Most cameras and other sensors record an image linearly. To prevent the non-linear behavior of the cathode ray tube from distorting a recorded image, it is necessary to apply a gamma value as an exponential correction factor before displaying the image.
Defined television standards require consumer television consoles to operate at a standardized gamma value. This allows commercial television stations to apply the gamma correction before a video signal is broadcast to eliminate the need for gamma correction in each individual console. Not all display systems, however, are subject to these standards. Computer monitors, for example, are not standardized to this broadcast gamma value, which can result in color distortion when displaying material from broadcast sources. In fact, most computer monitors completely ignore the effects of gamma distortion, leading to discrepancies in color between material on different displays.
The correction of the gamma value is further complicated by the translation of a signal between color domains. Television signals are typically transmitted using the YCbCr model, which represents a pixel according to its intensity Y, and two color difference values, Cb and Cr. Computation of a gamma corrected signal becomes prohibitively complex when the signal is represented in the domain defined by the Y, Cb, and Cr values. Thus, prior to gamma correction, it is necessary to transform the signal into an RGB color space domain, in which a pixel is represented by values indicating its content of red, green, and blue.
Further, even where an input signal is already in the RGB color space domain, it may be necessary to correct it for chromatic aberrations caused by differing RGB systems associated with the acquiring device (e.g., a camera or scanner) and the display. There are a number of different standards for the RGB color space domain, such as the National Television System Committee (NTSC) standard used by the United States, Japan, and a number of other nations, and the European Broadcasting Union (EBU) standard, used mainly by nations of Europe. Each standard defines the primary colors (i.e., red, green, and blue), differently, such that a display designed to receive signals broadcast according to the NTSC standard will show significant chromatic aberrations when receiving signals in the EBU standard.
Past solutions have included the use of look-up tables for correcting for differences in the gamma values and chromaticity values of displays. While this is effective for applications with a fixed input gamma value and a fixed output gamma value, it does not allow the user to change the gamma adjustment to suit personal preference or to adjust for inputs with varying associated gamma values. It would be inefficient to include a range of look-up table values for every possible degree of gamma correction that might be desired by a user. Thus, an efficient and cost-effective video correction system capable of adjusting to a wide range of input gamma values would be desirable.